Localization of CO2 sensor related to the inhibition of the bullfrog respiration

CO2 sensitivity in the airways and the general skin surface of the bullfrog under urethane anesthesia or without anesthesia was investigated. Pressure in the buccal cavity as well as blood pressure in the sciatic artery were measured with a differential or a straingauge transducer. Air containing 2--14% CO2 was introduced into the regions as given below. (1) The nose and the body surface, both regions were separated from each other and independently exposed to CO2. (2) The larynx-lungs, the buccal cavity-lungs, the naso-buccal cavity separated from the airways at the glottis, and the internal or external nares, respectively. By analyzing the CO2 sensitivity of the respective regions mentioned above, both the nasal mucosa and the skin surface were found to be responsible for the respiratory inhibition by CO2. Sectioning both the olfactory and the trigeminal nerves abolished the CO2-induced inhibition mediated by the nasal mucosa and electrical stimulation of the proximal cut end of these nerves inhibited respiration. These findings suggested the existence of afferent reflex pathways from the nasal mucosa by these cranial nerves. Significance of this CO2-induced reflex was discussed.